A charged clock
A clock face has negative charges fixed at the position of the corresponding numerals on the dial. The clock hands do not disturb the net field due to point charges. At what time does the hour hand point in the same direction as electric field at the centre of the dial?
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Suppose your answer is 4:45, then type 4.45 as your answer.
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A.M and P.M has no significance here.
The answer is 9.30.
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1 solution
The fields due to the charges at 1 o'clock and 7 o'clock cancel each other in part, leaving a field in the 7 o'clock direction equivalent to a − 6 q charge. The same is true for 2 o'clock/8 o'clock, and so on.
Therefore the field at the center can be viewed as the sum of six equal contributions, in the directions 7 o'clock, 8 o'clock, etc. through 12 o'clock.
Because these six contributions are symmetric, their sum lies along their axis of symmetry, that is, halfway between the 9 and 10 o'clock positions. Thus the field points in the 9 : 3 0 direction.